On an IBM 3278 display station, data fields extend horizontally
across the screen and are separated from one another by an attribute character.
Data fields may occupy one or more contiguous lines on the screen.
SNA IMF intrinsics read a screen one row at a time, beginning at
the upper left corner and moving along each row from left to right, looking
for a field attribute character. The first field attribute character found
causes the following field to be labeled field number one; the field following
the next attribute character is labeled field number two, and so
on as SNA IMF scans down the screen. If an attribute character is not
in the very first character position on the screen, then the first characters
of data belong to the last field on the screen; that is, the last field
wraps the screen.
SNA IMF intrinsics number rows in the screen 0 through 23
and columns 0 through 79. The IBM screen layout numbers rows 1 through 24
and columns 1 through 80. Figure 2-1 “Comparing SNA IMF and IBM Screen
Layouts” compares the
way SNA IMF and IBM number the rows and columns on a screen.
Figure 2-1 Comparing SNA IMF and IBM Screen
Layouts
An attribute is not considered part of the data field. The
field-oriented intrinsics never return the attribute when they return
field data. The data field begins with the first position following
the attribute character. This position is defined as the location
of the first data character in the field; here, offset = 0. When the READFIELD intrinsic returns data from a field, embedded
and leading null characters are returned with data characters, but
trailing null characters are not returned. This means that the actual
field length parameter count does not include trailing null characters.
